In electrophotography photoconductive materials are used to form a latent electrostatic charge image that is developable with finely divided coloring material, called toner.
The developed image can then be permanently affixed to the photoconductive recording material, e.g. a photoconductive zinc oxide-binder layer, or transferred from the photoconductor layer, e.g. a selenium or selenium alloy layer, onto a receptor material, e.g. plain paper and fixed thereon. In electrophotographic copying and printing systems with toner transfer to a receptor material the photoconductive recording material is reusable. In order to permit rapid multiple printing or copying, a photoconductor layer has to be used that rapidly loses its charge on photo-exposure and also rapidly regains its insulating state after the exposure to receive again a sufficiently high electrostatic charge for a next image formation. The failure of a material to return completely to its relatively insulating state prior to succeeding charging/imaging steps is commonly known in the art as "fatigue".
The fatigue phenomenon has been used as a guide in the selection of commercially useful photoconductive materials, since the fatigue of the photoconductive layer limits the copying rates achievable.
A further important property which determines the suitability of a particular photoconductive material for electrophotographic copying is its photosensitivity, which must be sufficiently high for use in copying apparatuses operating with the fairly low intensity light reflected from the original. Commercial usefulness also requires that the photoconductive layer has a spectral sensitivity that matches the spectral intensity distribution of the light source e.g. a laser or a lamp. This enables, in the case of a white light source, all the colors to be reproduced in balance.
Known photoconductive recording materials exist in different configurations with one or more "active" layers coated on a conducting substrate and include optionally an outermost protective layer. By "active" layer is meant a layer that plays a role in the formation of the electrostatic charge image. Such a layer may be the layer responsible for charge carrier generation, charge carrier transport or both. Such layers may have a homogeneous structure or heterogeneous structure.
Examples of active layers in said photoconductive recording material having a homogeneous structure are layers made of vacuum-deposited photoconductive selenium, doped silicon, selenium alloys and homogeneous photoconducting polymer coatings, e.g. of poly(vinylcarbazole) or polymeric binder(s) molecularly doped with an electron (negative charge carrier) transporting compound or a hole (positive charge carrier) transporting compound such as particular hydrazones, amines and heteroaromatic compounds sensitized by a dissolved dye, so that in said layers both charge carrier generation and charge carrier transport take place.
Examples of active layers in said photoconductive recording material having a heterogeneous structure are layers of one or more photosensitive organic or inorganic charge generating pigment particles dispersed in a polymer binder or polymer binder mixture in the presence optionally of (a) molecularly dispersed charge transport compound(s), so that the recording layer may exhibit only charge carrier generation properties or both charge carrier generation and charge transport properties.
According to an embodiment that may offer photoconductive recording materials with particularly low fatigue a charge generating and charge transporting layer are combined in contiguous relationship. Layers which serve only for the charge transport of charge generated in an adjacent charge generating layer are e.g. plasma-deposited inorganic layers, photoconducting polymer layers, e.g. on the basis of poly(N-vinylcarbazole) or layers made of low molecular weight organic compounds molecularly distributed in a polymer binder or binder mixture.
Useful charge carrier generating pigments (CGM's) belong to one of the following classes:
a) perylimides, e.g. C.I. 71130 (C.I.=Color Index) described in DBP 2 237 539; PA1 b) polynuclear quinones, e.g. anthanthrones such as C.I. 59 300 described in DBP 2 237 678; PA1 c) quinacridones, e.g. C.I. 46 500 described in DBP 2 237 679; PA1 d) naphthalene 1,4,5,8-tetracarboxylic acid derived pigments including the perinones, e.g. Orange GR, C.I. 71 105 described in DBP 2 239 923; PA1 e) tetrabenzoporphyrins and tetranaphthaloporphyrins, e.g. H.sub.2 -phthalocyanine in X-crystal form (X-H.sub.2 Pc) described in U.S. Pat. No. 3,357,989, metal phthalocyanines, e.g. CuPc C.I. 74 160 described in DBP 2 239 924, indium phthalocyanine described in U.S. Pat. No. 4,713,312 and tetrabenzoporphyrins described in EP 428 214A; and naphthalocyanines having siloxy groups bonded to the central metal silicon described in published EP-A 243,205; PA1 f) indigo- and thioindigo dyes, e.g. Pigment Red 88, C.I. 73 312 described in DBP 2 237 680; PA1 g) benzothioxanthene derivatives as described e.g. in Deutsches Auslegungsschrift (DAS) 2 355 075; PA1 h) perylene 3,4,9,10-tetracarboxylic acid derived pigments including condensation products with o-diamines as described e.g. in DAS 2 314 051; PA1 i) polyazo-pigments including bisazo-, trisazo- and tetrakisazo-pigments, e.g. Chlordiane Blue C.I. 21 180 described in DAS 2 635 887, trisazo-pigments, e.g. as described in U.S. Pat. No. 4,990,421 and bisazo-pigments described in Deutsches Offenlegungsschrift (DOS) 2 919 791, DOS 3 026 653 and DOS 3 032 117; PA1 j) squarylium dyes as described e.g. in DAS 2 401 220; PA1 k) polymethine dyes; PA1 l) dyes containing quinazoline groups, e.g. as described in GB-P 1,416,602 according to the following general formula: ##STR1## in which R and R.sup.1 are either identical or different and denote hydrogen, C.sub.1 -C.sub.4 alkyl, alkoxy, halogen, nitro or hydroxyl or together denote a fused aromatic ring system; PA1 m) triarylmethane dyes; and PA1 n) dyes containing 1.5-diamino-anthraquinone groups, PA1 o) inorganic photoconducting pigments e.g. Se, Se alloys, As.sub.2 Se.sub.3, TiO.sub.2, ZnO, CdS, etc. PA1 a) dicyanomethylene and cyano alkoxycarbonylmethylene condensates with aromatic ketones such as 9-dicyanomethylene-2,4,7-trinitro-fluorenone (DTF); 1-dicyanomethylene-indan-1-ones as described in published European application 0 537 808 with the formula: ##STR2## wherein R.sup.1, R.sup.2, X and Y have the meaning described in said published European application; compounds with the formula: ##STR3## wherein A is a spacer linkage selected from the group consisting of an alkylene group, including a substituted alkylene group, a bivalent aromatic group including a substituted bivalent aromatic group; S is sulfur, and B is selected from the group consisting of an alkyl group, including a substituted alkyl group, an aryl group including a substituted aryl group as disclosed in U.S. Pat. No. 4,546,054, and 4-dicyanomethylene 1,1-dioxo-thiopyran-4-one derivatives as disclosed in U.S. Pat. No. 4,514,481 and U.S. Pat. No. 4,968,813 e.g. ##STR4## b) derivatives of malononitrile dimers as described in EP-A 0,534,004; c) nitrated fluorenones such as 2,4,7-trinitrofluorenone and 2,4,5,7-tetranitrofluorenone; PA1 d) substituted 9-dicyanomethylene fluorene compounds as disclosed in U.S. Pat. No. 4,562,132; PA1 e) 1,1,2-tricyanoethylene derivatives. PA1 i) interfacial mixing between the CGL and the CTL resulting in CGM-doping of the CTL and CTM-doping of the CGL causing charge trapping; PA1 ii) charge trapping in the CGL; PA1 iii) poor charge transport in the CGL; PA1 iv) poor charge transport blocking properties in the absence of a blocking layer.
Organic charge carrier transporting substances may be either polymeric or non-polymeric materials.
Preferred non-polymeric materials for negative charge transport are:
The choice of binder for the charge generating layer (CGL) for a given charge generating pigment (CGM) and a given charge transport layer (CTL) has a strong influence on the electro-optical properties of the photoreceptors. One or more of the following phenomena can have a negative influence on the electro-optical properties of the photoconductive recording material:
Interfacial mixing between the CGL and the CTL can be avoided by using a CGL-binder or binders, which is/are insoluble in the solvent used for dissolving the CTL-binders in which CTM's exhibit optimum charge transport properties is limited as is the range of solvents in which efficient CTM's are soluble. The range of solvents in which both CTL-binders and CTM's are soluble is extremely narrow and often limited to chlorohydrocarbons such as methylene chloride. Methylene chloride is an extremely powerful solvent and the range of CGL-binders which is totally insoluble in methylene chloride is extremely limited, unless the CGL-binder is crosslinked in a subsequent hardening process.
Hardening is considered here as a treatment which renders the binder of a charge generating layer of the photoconductive recording material insoluble in methylene chloride.